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Gray Iron­A Unique Engineering Material

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Gray iron is the most versatile of all foundry metals. The high carbon content is responsible for ease of melting and casting in the foundry and for ease of machining in subsequent manufacturing. The low degree or absence of shrinkage and high fluidity provide maximum freedom of design for the engineer. By suitable adjustment in composition and selection of casting method, tensile strength can be varied from less than 20,000 psi to over 60,000 psi and hardness from 100 to 300 BHN in the ascast condition. By subsequent heat treatment, the hardness can be increased to H Rc 60 | Although there are differences in the rigidity of machine tools, some deflection always occurs. For the same bearing housing casting mentioned above, it was found that variations in Brinell hardness affected the finished bore diameter. The machine shop had previously classified finished bores into three size ranges. It was thought that this gaging operation could be eliminated by reducing machining tolerances to "0.00025 in. on a 2.5 to 3.0-in.-diameter bore. The castings were machined dry on a single spindle automatic lathe. Two cuts were taken, and the rough and finish tools were mounted in tandem on the same carrier. The operator was asked to set the machine to obtain the correct dimension and told not to change the setting regardless of bore diameter. Both the operator and inspector grouped castings by bore diameters being undersize, within range, and oversize. Specimens from the three groups were subjected to intensive examination and one of the factors appeared to be hardness as shown in Fig. 7.